The present invention relates to the field of forming and shaping of materials by various processes, including but not limited to cutting (e.g., shaping parts by removing excess material in the form of chips) and other types of machining, and more particularly improving surface finish and surface integrity of metals and other engineering materials (e.g., polymers and various types of composite materials) formed and shaped through such processes by utilizing cryogenic cooling and other types of treatments, including but not limited to heat treatment, chemical treatment, and mechanical treatment.
As used herein, the term “cutting” includes but is not limited to the following operations: turning, boring, parting, grooving, facing, planning, milling, drilling, and other operations which generate continuous chips or fragmented or segmented chips. The term cutting does not include: grinding, electro-discharge machining, or high-pressure jet erosion cutting, i.e., abrasive operations generating very fine chips that are not well defined in shape, e.g., dust or powder.
The term “integrity,” as used herein, relates to quality, and more specifically to the desired state of residual stresses in the processed work surface, dimensional accuracy affected by wearing tools, and/or the absence of artifacts or other undesired alterations of surface that often result from the conventional forming or shaping processes.
There is a need in the manufacturing industries to produce more parts or products faster, i.e., to produce each part or product faster and without increasing the cost per part or comprising part quality. More specifically, there is a need for improved methods which minimize the number and/or the extent of manufacturing steps required to produce a specific, good quality part or product, such as soft roughing, typically carried out before heat treatment, or finish grinding and polishing/honing, typically carried out following heat treatment, or cleaning steps, usually carried out on parts, machine tools, and in a work environment due to the contamination caused by conventional machining fluids. Moreover, there is an industrial interest in eliminating or minimizing the extent of various peening, burnishing, deburning, and localized deep-rolling operations completing the forming or machining process cycle and used, in the case of many metallic products, to enhance the mechanical surface integrity or remove detrimental tensile stresses produced during forming or machining. There also is a need for improved methods to accelerate forming and machining operations, minimize capital expenses, e.g., the number of machine tools required to reach specific production targets, and/or reduce the cost of tooling and associated consumables.
U.S. Pat. No. 5,878,496 (Liu, et al.) discloses a method for reducing the number of machining steps while producing hard work parts with an acceptable surface finish by an experimentation and modeling-based manipulation of conventional machining parameters including tool feedrate and nose radius. The patent does not, however, teach how to improve productivity, increase cutting tool life, or reduce the roughness of a work surface.
There exists a relatively large body of prior art publications pertaining to some form of cryogenic spraying or jetting to eliminate cleaning operations, effect productivity of various types of cutting tools, and/or prevent undesired microstructural changes within machined surfaces. See, for example, WO02/096598A1 (Zurecki, et al.), WO99/60079 (Hong), U.S. Pat. Application Nos.: 2003/0145694A1 (Zurecki, et al.) and 2003/0110781A1 (Zurecki, et al.), and U.S. Pat. No.: 5,901,623 (Hong), U.S. Pat. No. 5,509,335 (Emerson), U.S. Pat. No. 4,829,859 (Yankoff), and U.S. Pat. No. 3,971,114 (Dudley). However, none of these publications nor the other prior art references discussed herein solve the problems or satisfy the needs discussed herein.
U.S. Pat. No. 5,761,974 (Wang, et al.) discloses the use of a cryogenic heat-exchanger in contact with the workpiece contacting edge of a cutting tool, whereby direct contact between the cryogenic fluid and the workpiece is avoided by use of the heat exchanger. U.S. Pat. No. 5,103,701 (Lundin, et al.) discloses that cryogenic freezing of an entire workpiece may result in an improvement of tool life when a sharp-edged diamond cutting tool is contacted with ferrous work materials. The methods taught by these two patents improve tool productivity, but the first method cannot effectively control work surface finish and integrity, and the second method requires extensive machine tool modifications that would be unacceptably expensive in most industrial applications.
U.S. Pat. No. 5,592,863 (Jaskowiak, et al.) discloses a method using cryogenic cooling to produce discontinuous chips from a continuous chip formed during machining of a polymer workpiece. By cooling the chip, rather than the cutting tool or the polymer workpiece, the method does not improve tool productivity or workpiece surface finish and integrity.
U.S. Pat. No. 6,622,570 B1 (Pervey, III) and U.S. Pat. Application No. 2002/0174528A1 (Prevey, III) disclose methods for eliminating undesired tensile stresses in a work surface that result from various manufacturing operations (e.g., turning) and for imparting desired, compressive stresses. Compressive residual stress in a work surface is known to enhance fatigue strength and fatigue life of product parts while reducing their sensitivity to stress corrosion cracking. An enhanced resistance to stress corrosion cracking and to other stress-accelerated forms of metal corrosion is invaluable to metal component producers and users. The key methods for correcting residual surface stress distribution (i.e., increasing its compressive component) include shot peening and laser peening, both of which are known to deteriorate or damage work surface finish and increase work roughness if applied to their fullest extent. Further illustration of this problem is found in U.S. Pat. No. 6,658,907 B2 (Inoue, et al.) and in U.S. Pat. No. 6,666,061 B2 (Heimann), the latter dealing with deep-rolling, another stress fixing method applied to the surface of manufactured parts. These four patent publications show two critical and still unsolved issues facing the industry: (a) a frequent need for an additional, expensive manufacturing step fixing residual surface stresses and following the forming or shaping steps, and (b) the present trade-offs between the surface finish and the compressive stress imparted during the stress fixing operations. Clearly, there is an unsatisfied need for an improved forming, shaping and machining technique which would enhance surface finish and compressive stresses at the same time without requiring additional manufacturing steps.
Others have reported that during the conventional, non-cryogenic turning of hard steels, a sharp cutting edge improves the surface finish and/or somewhat enhances the desired compressive residual stresses, while a rounded or honed edge, preferred from the tool-life and productivity standpoint, makes the workpiece surface rougher and/or less compressed. J. D. Thiele and S. N. Melkote, Effect of cutting edge geometry and workpiece hardness on surface generation in the finish hard turning of AISI 52100 steel, Journal of Materials Processing Technology, 94 (1999) 216-226; and F. Gunnberg, “Surface Integrity Generated by Hard Turning,” Thesis, Dept. of Product Development, Chalmers University of Technology, Goteborg, Sweden, 2003. The impact of the honed edge geometry on work surface finish was observed to lessen with increasing work material hardness, but no conclusions were drawn regarding the prospect of controlling surface finish and integrity by modifying work surface hardness before or during machining operations while maintaining an acceptable tool life and high productivity.
Also, experimental roughness data which has been reported for very similar machining conditions underlined the tentative nature of material hardness effect suggested by Thiele and Melkote, showing that the roughness increases whenever the work hardness increases. See, T. Ozel, Tsu-Kong Hsu, and E. Zeren, Effects of Cutting Edge Geometry, Workpiece Hardness, Feed Rate and Cutting Speed on Surface Roughness and Forces in Finish Turning of Hardened AISI H13 Steel, International Journal of Advanced Manufacturing Technology (2003).
Thus, the prior art offers only fragmented and incomplete, if not contradicting, solutions to the industrial needs discussed above, and demonstrates the need for a more comprehensive method for reducing manufacturing steps and costs while improving work surface finish and integrity. Specific areas that require a single, comprehensive solution include (a) effectiveness of cooling and hardening of cutting tools during machining using cryogenic jetting, which is preferred for its ability to reduce tool wear and costs, increase production rates, and eliminate cleaning steps from the manufacturing process, (b) application of cryogenic jetting to minimize roughness and maximize compressive stresses of work surface produced during machining so that no additional finishing steps are required, and (c) further modifications of work material properties before and during cutting that minimize machined surface roughness and, thus, eliminate the need for finish grinding steps.
It is desired to have a method and an apparatus for improving the surface finish and integrity of a workpiece which satisfy the above needs and address the problems discussed herein.
It is further desired to have a method and an apparatus for improving the surface finish and integrity of a workpiece which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is still further desired to have a method and an apparatus for forming or shaping a workpiece which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is also desired to have a method and an apparatus for manufacturing finished parts and products which would eliminate one or more steps or elements required in prior art manufacturing processes and systems.